What is vapour pressure?
Partial atmospheric pressure exerted by the vapours of a liquid
* When vapour pressure is equal to the atmospheric pressure, this is the definition of boiling point
* The more vapour (i.e. the more volatile) the more there is to burn (energy for combustion)
What are flammability limits?
- Because our combustion reactions need to be in the right stoichiometry, combustion is only possible within certain regions
- In a closed system, the explosive limit will be the same as the flammability limit
- In an open system, other factors (such as turbulence) can interfere
- Temperature alters the vapour pressure and so the likelihood of fire
Lower flammability limit
- The lowest concentration of flammable gas in air that can sustain combustion
- Below this is fuel/lean/oxygen rich
- No ignition will occur below the LEL no matter how much energy is given
- Not enough fuel
- If volatilisation is promoted, this will bring it above the LEL which will allow for combustion
Upper flammability limit
- The highest concentration of flammable gas in air that can sustain combustion
- Gas will burn but will ususally be incomplete combustion above the UEL
- Not enough oxygen above this to sustain a reaction (fuel rich)
- Slow but not ideal combustion
- As the fuel gets used up we will reach between the flammability range
How to work out the stoichiometric concentration of a fuel?
- Balance the combustion equation and work out the molecular ratio between the fuel and oxygen (O2) - x : y
- Oxygen comprises 21% of air, x : y(100/21)
How to work out the lower flammability limit?
Divide the stoichiometric concentration by 2
* LEL tends to be half of the calculated stoichiometric conc
What will a mixture with a low ratio of vapour to air cause?
Lean explosion
* very powerful, but brief and may not result in a subsequent fire
What does a mixture with a high ratio of vapour to air cause?
Flaming, rolling explosion often followed by a fierce fire
What are the flammability limits?
Definition
The range of concentrations of fuel vapour capable of burning in air (21% oxygen)
What will happen at other % of oxygen in air?
- If the concentration falls below 15% the rate of combustion will decrease
- At low enough oxygen concentrations, the flames will die out and be replaced by smoulder (or glowing) combustion at the surface of solids
- The oxygen conc depends on the flammability limits of the fuel and the temperature of the combustion gases
What is the flash point?
- The minimum temperature at which the vapour produced by a liquid can be ignited momentarly in air
- The temp at which the vapour conc reaches the LFL
What factors influence the flash point?
- The heat generated from enthalpy of combustion
- The heat capacity of the combustion products
- The rate of heat loss from the flame by radiation
- The kinetic rate production of more vapour
What is the fire point?
- The minimum temperature at which sufficient vapour is produced by liquid to sustain combustion after ignition in air
- Usually a few degrees higher than the flash point
What factors influence the fire point?
The volatility of the liquid
* vapour must be produced at a rate sufficient to maintain concentration above the lower flammability limit
What is best for an arsonist in regards to flash and fire point and why?
Low fire and flash point
* Low flash point temps correlates with high vapour pressure (high volatility)
How do you measure the flash point and fire point?
- Place test fuel into a container
- Liquid fuel is slowly and uniformly heated with its temp measured
- Periodically as the temp is increased, an ignition source is inserted into the container
- The lowest temp where a flash occurs is defined as the flash point temp
Why is it difficult to measure the flash and fire point?
- The vapour must be at the measured temperature
- Also requires the presence of air
- Difficult to get defined experimental condition - even harder to represent real life scenarios
Define auto-ignition or spontaneous ignition temperature
The temp at which the fuel will ignite without any additional source
* Reflects activation energy more than volatility
What is the unit for heats of combustion?
kJ kg^-1
What is aliphatic?
- Non-aromatic
- Cyclic
- Have conjugation
What is aromatic?
- Delocalised conjugated rings
- 4n + 2 pi electrons where n is carbons
Examples of hydrocarbon mixtures
- Petrol
- Paraffin
- Diesel
- Petroleum ether
- Benzine
- White spirit
- Turpentine
- Methylated spirits
- Paint thinners
How liquids behave in fires
- Viscous liquids spread less far than those which flow more easily
- Porous surfaces absorb liquids and make smaller pools, but evaporation is faster due to wick effect
- Porous surfaces can provide samples for lab analysis
- Latent heat of evaporation protects the area under the pool from damage to some extent
Examples of gas fuels
- Methane
- Liquid petroleum gas
- Aerosol products
